Steam reforming, partial oxidation and oxidative steam reforming for hydrogen production from ethanol over cerium nickel based oxyhydride catalyst

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Steam reforming, partial oxidation and oxidative steam reforming for hydrogen production from ethanol over cerium nickel based oxyhydride catalyst Cyril Pirez a,b,1 , Wenhao Fang a,b,1 , Mickaël Capron a,b , Sébastien Paul b,c , Hervé Jobic d , Franck Dumeignil a,b,e , Louise Jalowiecki-Duhamel a,b,∗ a

Université Lille Nord de France, 59000 Lille, France Centre National de la Recherche Scientifique UMR 8181, Unité de Catalyse et Chimie du Solide, UCCS, 59655 Villeneuve d’Ascq Cedex, France c Ecole Centrale de Lille, 59655 Villeneuve d’Ascq, France d IRCELyon Institut de Recherches sur la Catalyse et I’Environnement de Lyon, 69626 Villeurbanne Cedex, France e Institut Universitaire de France, Maison des Universités, 103 Boulevard Saint-Michel, 75005 Paris, France b

a r t i c l e

i n f o

Article history: Received 17 July 2015 Received in revised form 12 October 2015 Accepted 20 October 2015 Available online xxx Keywords: Ethanol Hydrogen Steam reforming Partial oxidation Oxidative steam reforming

a b s t r a c t H2 production from ethanol was studied on a cerium nickel based (CeNi1 OY ) catalyst in presence of water (SRE), in presence of oxygen with different concentrations (POE) and in presence of oxygen and water (OSRE). The influence of different parameters was analyzed, such as the reaction temperature, reactants concentration (oxygen/ethanol/water), and in situ pre-treatment in H2 . At low temperature a high activity i.e., ethanol conversion and H2 formation can be obtained when the solid is previously in situ treated in H2 at 250 ◦ C i.e., in the oxyhydride form. Adding O2 allows increasing ethanol conversion at low temperature. Different physicochemical techniques, including Inelastic Neutron Scattering (INS), were used to characterize the catalyst. An active site based on the formation of anionic vacancies and a mechanism involving a heterolytic abstraction of a hydride species from ethanol are envisaged. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, increasing attention is being paid to pollutionrelated environmental and public health problems, it is important to find a way for obtaining “green” energy. A hydrogen economy coming from renewable energy sources would lead to high benefits at the world level [1–2]. Today hydrogen is mainly produced from fossil fuels and while fossil fuel is not a sustainable energy source, one highly attractive route for hydrogen production is catalytic transformation of bio-ethanol, obtained from transformation and fermentation of biomass. In theory, hydrogen production from biomass or biomass derived liquids can be a neutral carbonemission process since all carbon dioxide produced can be recycled back to plants, and because of its low toxicity and ease of deliverability, ethanol lends itself very well to a distributed-production strategy [3–11]. Besides, the wide-spread application of fuel cells

∗ Corresponding author at: UCCS, CNRS, 59655 Villeneuve d’Ascq Cedex, France. Fax: +33 (0) 3 20 33 65 61. E-mail address: [email protected] (L. Jalowiecki-Duhamel). 1 These authors contributed equally to this work.

become closer to reality, so increased attention is focused on hydrogen production technologies [12–16]. Hydrogen may be generated from ethanol by different technologies, as previously described in different reviews [3–11]: steam reforming (SRE) [Eq. (1)] [17–27], partial oxidation (POE) [Eq. (2)] [28–38], and oxidative steam reforming (OSRE) [39–47], including autothermal reforming (ATRE) [48–51] reactions. Moreover, the different technologies have been already compared on some catalysts [50–52]. The endothermic SRE reaction extracts more hydrogen atoms from ethanol and water molecules and provides a high hydrogen yield, which represents an important advantage in hydrogen-production applications. However, SRE is highly endothermic reaction and therefore, high operation temperatures are necessary, so additional energy supply is needed, which leads to high capital and operation costs as well as environmental degradation. The reforming reaction is generally carried out at high temperature (>600 ◦ C). An alternative approach is partial oxidation of ethanol (POE), an exothermic reaction that exhibits fast start up and response times while potentially offering a more compact reactor design, desirable features for mobile fuel cell applications. Therefore, one alternative way of supplying heat to the reforming of ethanol system is to add oxygen or air to the feedstock (OSRE, ATRE)

http://dx.doi.org/10.1016/j.apcata.2015.10.035 0926-860X/© 2015 Elsevier B.V. All rights reserved.

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and simultaneously to burn a portion of ethanol. POE can be performed with different oxygen concentrations as reported in Eqs. (2) and (3), and the reaction is exothermic for O2 /ethanol molar ratio higher than 0.5, while with this ratio the reaction is endothermic. C2 H5 OH + 3H2 O → 6H2 + 2CO2

(1)

H ◦ 298 = +174 KJ mol−1 C2 H5 OH + 1.5O2 → 3H2 + 2CO2

(2)

H ◦ 298 = −545 KJ mol−1 C2 H5 OH + 0.5O2 → 2CO + 3H2

(3)

H ◦ 298 = +14 KJ mol−1 Nevertheless, the major barriers to all of these technologies are the by-products formation and catalytic deactivation. So, enormous research efforts have been done to develop cheap, highly active, selective and resistant catalysts and/or electrocatalysts, as ethanol, can be sent directly to a fuel cell. The reactions have been studied over noble metals as well as transition metals with different metals and numerous supports [5–9], and among them nickel and cerium based catalysts have been tested [17,18]. OSRE interested the scientists because adding O2 show beneficial effects [53]. Although the POE reaction could be performed at relatively lower temperature, the high exothermicity of the reaction [Eq. (2)] could lead to hot-spots and deactivation of the catalyst. It is therefore of great interest to compare between SRE, POE and OSRE on exactly the same catalyst, and analyze the influence of different parameters. In the laboratory, different nickel based mixed oxides were studied for hydrogen production from ethanol in SRE [54–58] and in OSRE [59,60]. It is known that the catalyst preparation method is of paramount importance, and we previously analyzed its influence over the cerium nickel based catalyst in SRE activity [56]. Therefore, it is of great interest to analyze the influence of the presence of oxygen using the same catalyst. Here, we report a comparative study on H2 formation activity over the CeNi1 OY catalyst from ethanol in presence of water (SRE), in presence of oxygen with different concentrations (POE) and in presence of oxygen and water (OSRE). The aim of this work was to develop a highly active, selective, stable and cost effective catalyst at low temperature whatever the reaction mixture, and to participate to the open debate on active site and mechanism.

at 250 ◦ C for 10 h. The water/ethanol mixture is pumped (with a HPLC pump) into a heated chamber and vaporized. In order to analyze the influence of the concentration of ethanol, different liquid flows of the ethanol–water mixture were used while the H2 O/EtOH molar ratio was always kept constant at 3. The liquid flows were between 0.01 and 0.10 mL min−1 . The ethanol/water/O2 /N2 gas stream (O2 –N2 flow: 60 mL min−1 ) is then fed to the reactor containing the (0.008–0.2 g) of catalyst. It has to be remarked that as the O2 –N2 flow is maintained constant, when the EtOH concentration is increased, the total theoretical flow is also increased. The gases at the outlet of the reactor were taken out intermittently with the aid of a sampler directly connected to the system and analyzed on-line by gas chromatography (TRACE GC ULTRA) equipped with a thermal-conductivity detector (TCD) and a flame ionization detector (FID). Solid carbon is formed among the products. For POE reaction, catalytic performances were conducted at atmospheric pressure with a quartz fixed-bed reactor (inner diameter 10 mm) fitted in a programmable oven, in the temperature range of 200–500 ◦ C. When noted, the catalyst was previously in situ treated in H2 at 250 ◦ C for 10 h. Ethanol is sent via a saturator and the partial ethanol pressure is controlled using a condensator, 4 mol% of EtOH was sent for almost all the experiments, while 4–20 mol% range of EtOH was studied when precised. It has to be remarked that as the O2 –He flow is maintained constant (O2 –He flow: 60 mL min−1 ), when the EtOH concentration is increased, the total theoretical flow is also increased. The O2 /ethanol molar ratio varies between 0.5 and 1.5. The ethanol/O2 /He gas stream is then fed to the reactor containing 0.2 g of catalyst diluted with SiC, and sandwiched between two layers of SiC. The gases at the outlet of the reactor were taken out intermittently with the aid of a sampler directly connected to the system and analyzed by FID and TCD gas chromatography. Reaction data were collected for each temperature. Solid carbon is formed among the products but not quantified. Appropriate blank runs have shown that under our experimental test conditions the contribution of the gas phase reaction is negligible. Reaction data were collected as a function of time and reported after at least about 5 h when the steady state was obtained, for each temperature. Catalytic performances were reported by ethanol conversion (XEtOH ), and products molar composition (Ci ) (dry basis), based on the following equations [Eqs. (4) and (5)]. XEtOH = Ci =

nEtOH,in − nEtOH,out × 100% nEtOH,in

ni × 100% products ni

(4) (5)

2. Experimental 2.3. Catalyst characterizations 2.1. Catalyst preparation The mixed oxide, denoted CeNi1 OY was prepared by coprecipitation of the corresponding hydroxides from mixtures of cerium and nickel nitrates (0.5 M) using triethylamine (TEA) as a precipitating agent. After filtration, the solid was dried at 100 ◦ C and calcined in air at 500 ◦ C for 4 h. The calcined compound is noted as fresh catalyst. 2.2. Catalytic performance For SRE and OSRE reactions, catalytic performances were conducted at atmospheric pressure with a quartz fixed-bed reactor (inner diameter 10 mm or 4 mm according to the mass of catalyst) fitted in a programmable oven, in the temperature range of 50–500 ◦ C. The catalyst was sieved after preparation and the solid with dimension between 250 and 500 ␮m was used for catalytic test. When noted, the catalyst was previously in situ treated in H2

The metal loadings were analyzed by ICP-MS technique from CNRS-Service Central d’Analyses and the molar ratio was then deduced. The BET surface area was measured by N2 physisorption at 77 K by using a Micromeritics TriStar II 3020 Surface-Area and Porosimetry analyzer. The sample was previously out-gassed under vacuum at 150 ◦ C for 3 h. Raman spectra were acquired on a Labram Infinity HORIBA JOBIN YVON Raman spectrometer using a visible laser with a wavelength of  = 532 nm at room temperature. H2 -TPR was performed on a Micromeritics Autochem II Chemisorption analyzer, and the H2 consumption was measured by a TCD detector. The sample was treated in the 5 vol.% H2 –95 vol.% Ar mixtures with a flow rate of 30 mL min−1 . The temperature was increased to 1000 ◦ C at a heating rate of 10 ◦ C min−1 . In situ XRD in H2 was performed on a Bruker D8 Advance type HT1200N X-ray diffractometer equipped with a fast detec-

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Fig. 1. Conversion of ethanol in SRE and gas phase products distribution at 450 C on the CeNi1 OY catalyst. Ac: acetaldehyde.

tor type VANTEC with a copper anticathode. A mixed gas of 3 vol.% H2 –97 vol.% Ar was employed with a heating rate of 10 ◦ C min−1 from room temperature to 450 ◦ C. Inelastic Neutron Scattering (INS) experiments were performed using the IN1 BeF spectrometer at ILL (Institut Laue Langevin, Grenoble). 36 g of solid were placed inside stainless steel containers, and the treatment in H2 (10 h) was performed using high purity gas. The scattering cross-section is much greater for hydrogen (80 barns) than for other elements (5 barns), therefore, INS emphasizes motions of hydrogen species. 3. Results and discussion Hydrogen production from ethanol SRE, POE and OSRE was investigated over a cerium nickel based mixed oxide catalyst (CeNi1 OY ). The influence of different parameters was analyzed, such as reaction temperature, water–oxygen–ethanol feed compositions, and in situ pre-treatment in H2 . 3.1. Steam reforming of ethanol (SRE) The efficiency of the CeNi1 OY catalyst was thoroughly studied by its capacity to produce hydrogen from SRE (H2 O/C2 H5 OH = 3) in diluted conditions (H2 O: 9 mol%; C2 H5 OH: 3 mol%) to avoid any problems due to volume variation when a high concentration of gases are produced (Fig. 1). At 450 ◦ C, the CeNi1 OY catalyst (0.05 g) is able to completely convert ethanol to H2 , CO2 and CH4 (almost no CO is observed), with a H2 yield of 3 mol molEtOH −1 [56]. It has to be noticed that decreasing the ethanol concentration allows still enhancing ethanol conversion [58]. On the present catalyst, the activity is high at 450 ◦ C and almost no variation is observed on the conversion with only a small conversion decrease when increasing EtOH concentration up to 14% (mol). In the meantime, the products distribution is influenced by the increase of EtOH concentration, with a decrease of H2 proportion (among the gas phase products) and an increase of CO and CH4 . However, it has to be noted that the products distribution is reported as obtained by online analysis, without any correction related to volume variation. The catalyst mass must be decreased down to 0.008 g to see a decrease of EtOH conversion, evidencing the high activity of this catalyst, with in such a case appearance of acetaldehyde (Fig. 1). Numerous different reactions can take place, and have been largely reviewed [5,7–10] however, acetaldehyde can be obtained by ethanol dehydrogenation [Eq. (6)] and decomposition of ethanol [Eq. (7)] and/or acetaldehyde lead to the formation of methane. C2 H5 OH → CH3 CHO + H2

(6)

C2 H5 OH → H2 + CH4 + CO

(7) 450 ◦ C

By increasing the reaction temperature from up to 650 ◦ C, CH4 formation gradually decreases from 20% down to zero,

3

Fig. 2. Conversion of ethanol in SRE (14% EtOH) and gas phase products distribution at 250 ◦ C on the CeNi1 OY catalyst (0.2 g) as a function of treatment temperature (TT ) in H2 . Ac: acetaldehyde, Eac: Ethylacetate.

while CO formation starts rising to about 13%, in agreement with methane reforming at high temperatures and CO transformation by a water gas shift at low temperatures. As a result, the yield of hydrogen significantly increases up to 4.6 mol molEtOH −1 at 650 ◦ C, while the catalyst exhibits considerable stability, even if some carbon is formed [56]. It is important to mention that working at high temperature water gas shift (WGS) equilibrium does not allow reaching the theoretical value of 6 mol molEtOH −1 . Moreover, it must be emphasized that the hydrogen production can be promoted by increasing the water partial pressure and the H2 O/C2 H5 OH molar ratio. Biswas et al. obtained at 600 ◦ C on a Ni/CeO2 –ZrO2 catalyst, in a reaction mixture with a high H2 O/EtOH ratio of 8, the highest value for H2 production (5.8 mol molEtOH −1 ) that has been reported in the literature, close to the theoretical value of 6 mol molEtOH −1 [18]. The other products formed were CO (0.47 mol molEtOH −1 ), CH4 (0.33 mol molEtOH −1 ) and CO2 (1.15 mol molEtOH −1 ). Therefore close to 75% of H2 were obtained with 6% of CO, 4% of CH4 and 15% of CO2 . As a matter of fact, thermodynamics predicts the equilibrium composition of reactants and products at different temperatures. Moreover, the increase of the H2 O/EtOH ratio, as well as the addition of helium as inert gas increases H2 production at low temperatures [61–62]. Besides, it is noticeable that the experimental results of H2 production are higher than the ones calculated by the thermodynamic analysis. It has been reported that there is a region of non-equilibrium in which higher H2 concentration can be obtained. Moreover, it has been also shown that the carbon formation involving different types of carbon species must also be taken into account in the thermodynamic calculation [61]. Hydrogen production from ethanol in the presence of water (H2 O/C2 H5 OH = 3) was also previously investigated over the series of cerium nickel CeNiX OY (0 < x ≤ 5) mixed oxide catalysts in harsh conditions (14 mol% EtOH) [54,55]. The influence of different parameters was analyzed, such as reaction temperature, Ni content and in situ pre-treatment in H2 . A stable activity i.e., ethanol conversion and H2 formation can be obtained at very low reaction temperature (200 ◦ C) when the solid is previously in situ treated in H2 at adequate temperature. When the CeNi0.4 OY [54] or CeNi0.5 OY [55] catalysts are previously in situ treated in H2 at 200 ◦ C, there is globally an increase of the conversion versus temperature with the existence of an optimum for a reaction temperature of 250 ◦ C. The influence of the in situ treatment in H2 at 200 ◦ C on the ethanol conversion obtained at 200 ◦ C and 250 ◦ C has been analyzed versus the Ni content of CeNiX OY compounds [54]. Moreover, the influence of the in situ treatment temperature TT has been analyzed on the CeNi0.7 OY compound, and an optimum of activity at 250 ◦ C has been shown for a TT of 270 ◦ C of the catalyst [55]. Fig. 2 shows the high influence of TT on the CeNi1 OY compound. The reaction is performed at a particularly low temperature of 250 ◦ C in harsh conditions (14 mol% of EtOH) maintaining the molar H2 O/C2 H5 OH ratio of 3. Clearly, the conversion and products distribution depend

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hence the performance obtained here with a H2 O/EtOH ratio of 3 is interesting as compared to the literature [58]. At this particularly low temperature, it could be expected that increasing Ni content would allow increasing conversion and ameliorating selectivity, if high dispersion of Ni could be maintained. As a matter of fact, it has been recently reported, in similar conditions, that total conversion of EtOH can be obtained with the expected products from SRE without formation of acetaldehyde on Ni–Mg–Al–O based catalyst involving higher Ni content [58]. Moreover, it has been shown that conversion of EtOH can be increased and acetaldehyde formation decreased by increasing Ni content on different nickel based catalysts (even if in such a case carbon formation increased also) [56,58]. Fig. 3. Conversion of ethanol in SRE and gas phase products distribution at 300 ◦ C as a function of time on stream on the CeNi1 OY catalyst (0.05 g) in situ pretreated in H2 at 250 ◦ C. EtOH/H2 O/N2 = 1/3/96. No CO is observed.

on the pre-treatment in H2 and an optimum TT of about 250 ◦ C is obtained. Increasing TT up to 450 ◦ C leads to a conversion decrease, and much particularly to an increase of carbon formation, even if a slight increase of H2 can be obtained. Without any pretreatment in H2 or with a low TT , the conversion remains very low and ethylacetate and acetaldehyde are mainly obtained in such conditions. All the results obtained show the high performance of the CeNi1 OY catalyst, in particular at low temperature. Numerous parameters are influencing the results obtained, in complement to the already reported water concentration and/or H2 O/EtOH ratio effects, such as the in situ pre-treatment in H2 that has to be performed at adequate temperature, as well as the ethanol concentration, the highest conversion being obtained with the lowest ethanol concentration. To be able to report high conversion at low temperature, low concentration of ethanol is favorable. As shown in Fig. 3, high conversion of EtOH can be obtained at only 300 ◦ C in highly diluted conditions (1% EtOH). After several hours, a stable EtOH conversion at about 60% is obtained on a low mass of catalyst (0.05 g), with the formation of 75% of H2 , 10% of CO2 , 10% of CH4 and 5% of acetaldehyde. No CO and no carbon are observed. It seems that the present catalyst allows totally transforming CO at low temperature of 300 ◦ C through WGS to H2 . As it is known that increasing water–ethanol ratio allows obtaining higher H2 yield,

3.2. Partial oxidation of ethanol (POE) 3.2.1. Effect of O2 /EtOH ratio The fresh CeNi1 OY catalyst has been tested in POE for different O2 /EtOH ratios between 200 and 480 ◦ C. Fig. 4 shows ethanol conversion and products distribution versus reaction temperature obtained with different oxygen/ethanol molar ratios: 0.56, 1 and 1.5. All the curves show the same tendency, the conversion increases with the temperature and reaches 100% at 350 ◦ C with a ratio of 1.5 and at 400 ◦ C with the other ratios, so at lower temperature with the highest concentration of O2 . Ethanol conversion increases more rapidly when the O2 /EtOH ratio increases. H2 appears at 300 ◦ C in the gas phase products, its molar fraction increases drastically between 300 and 350 ◦ C, and then it increases slightly when increasing temperature. H2 formation follows almost the same trend with the O2 /EtOH ratios, however, it is lower with the highest O2 /EtOH ratio, and a slight optimum could be seen with the ratio 1 at 440 ◦ C, with 68% of H2 among the gas phase products. Even if ethanol conversion is complete at 350 or 400 ◦ C depending on O2 concentration, oxygen conversion is complete at 300 ◦ C. At low temperature (200–300 ◦ C), the main products observed are acetaldehyde, CO2 , acetone and ethylacetate, depending on the O2 content; acetaldehyde, being the main product at the lowest temperature analyzed (200 ◦ C). The formation of CO2 presents an optimum; it reaches about 70% with the highest concentration of O2 at 300 ◦ C. The high formation of CO2 is coherent with Eq. (2), however, hydrogen started only to be observed at this

Fig. 4. Ethanol conversion and products distribution in POE over fresh CeNi1 OY catalyst (0.2 g) in presence of different O2 /EtOH ratios: (䊐) 0.56, () 1, () 1.5.

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Fig. 5. Ethanol conversion and products distribution in POE over fresh CeNi1 OY catalyst (0.2 g) in presence of an O2 /EtOH ratio of 1.5. The oven temperature () and the measured temperature () are also reported in (a).

temperature, may be because the catalyst consumes hydrogen and get partially reduced. CO is found in much lower concentrations, whatever the O2 concentration; it presents an optimum at 350 ◦ C. Whatever the O2 /EtOH ratio, ethylacetate is found at 17% at 200 ◦ C and it decreases with temperature to disappear at 350 ◦ C, while acetone presents an optimum at about 7% at 300 ◦ C. Acetaldehyde and ethylacetate disappear at 400 ◦ C while methane is formed at temperatures higher than 300 ◦ C. As discussed for SRE reaction, several competing reactions can exist such as ethanol dehydrogenation and decomposition. Moreover, in presence of O2 , partial oxidation reactions take place. POE may be even considered as a combination of partial oxidation to syngas coupled with CO oxidation. Furthermore, oxidative dehydrogenation to acetaldehyde could occur in parallel with POE, as already previously reviewed [5,7–9]. 3.2.2. Effect of reactants concentration The fresh CeNi1 OY catalyst has been also tested in POE with the O2 /EtOH ratio of 1.5 varying the concentration of EtOH between 4 and 20 mol.%, and consequently also the O2 concentration as here the O2 /EtOH ratio is maintained constant (Fig. 5). The conversion of EtOH is found relatively stable versus EtOH concentration, reaching almost 60%, however, it is important to note that to get the same reaction temperature, the oven temperature had to be lowered (Fig. 5a). With an oven fixed at 300 ◦ C and an EtOH concentration of 4%, there is almost no variation of temperature between the oven and reaction temperatures. However, there is almost 100 ◦ C of difference between the oven and reaction temperatures when the EtOH concentration is of 20% (oven temperature at about 220 ◦ C in order to get a reaction temperature of 300 ◦ C). Therefore, the same conversion is obtained at the same reaction temperature, however, with a higher total theoretical flow when EtOH concentration is increasing. So the catalyst can be seen as more active in high EtOH concentration in comparison to the results obtained with low EtOH concentration, while the inverse has been observed previously in SRE. H2, CH4 , CO and acetaldehyde increase globally with increasing EtOH concentration while CO2 and acetone decreases. Ethylacetate is found in low quantity. 3.2.3. Effect of in-situ pretreatment in H2 The influence of the in situ pretreatment in H2 at 250 ◦ C of the CeNi1 OY catalyst has been studied with the O2 /EtOH ratio of 1.5 (Fig. 6). This treatment leads to a complete ethanol conversion in POE at 300 ◦ C, so at 50 ◦ C lower than without pretreatment, and also to the apparition of H2 at 250 ◦ C (Fig. 6a), so at 100 ◦ C lower compared to fresh catalyst. Therefore, the activation in H2 has a beneficial effect on ethanol conversion and H2 formation at low temperature in POE. While O2 conversion is the same for the

treated and fresh catalyst. At temperatures higher or equal to 350 ◦ C ethanol and O2 conversions and products distribution (unless for CH4 ) do not seem to depend on the activation of the catalyst. However, one must note that the conversion being total it may be not possible to see differences. The main differences in the gas phase products distribution are observed at low temperature, mainly for H2 , CO and CH4 that are favored at lower temperature on the treated in H2 catalyst. Clearly, the treatment in H2 at 250 ◦ C leads to a beneficial effect on H2 production at low temperature. CH4 and CO are observed at temperatures higher than 250 ◦ C. Hence, ethanol [Eq. (7)] and/or acetaldehyde are decomposed to CO and CH4 at lower temperature over the pretreated in H2 catalyst. Moreover, CH3 CHO obtained by dehydrogenation of ethanol [Eq. (6)], decreases more drastically with reaction temperature on the H2 pretreated catalyst. So the pretreatment in H2 generates active sites able to perform the corresponding reactions at low temperature. Formation of acetone and ethylacetate, not reported here, diminish on the H2 treated catalyst. At 300 ◦ C their molar fractions is of about 1%, showing that the active sites related to the formation of these products are suppressed. Besides, over the fresh catalyst, up to 300 ◦ C acetaldehyde is observed but not H2 , so H2 that should be produced from ethanol by dehydrogenation reaction [Eq. (6)] is certainly consumed by the solid leading to a partially reduced compound, explaining the beneficial effect observed due to the pretreatment in H2 of the catalyst. Hydrogen species already present in the solid allow the H2 formation (desorption) at low temperature even in presence of oxygen. The partial oxidation of ethanol (POE) reaction for H2 production has been much less reported compared to SRE, and in particular POE using an O2 /EtOH = 1.5 [Eq. (2)], some more papers focusing on lower O2 /EtOH ratios [Eq. (3)]. Depending on the nature of metal catalyst used, the pretreatment applied and the reaction operating conditions employed, different products can be observed [5,7–9]. Moreover, it is clearly shown in the present study that even on exactly the same catalyst, different pretreatment and conditions applied lead to different results evidencing the versatility of a catalyst. The POE to H2 and CO2 has been reported at low temperatures, (300–400 ◦ C) using an O2 /EtOH molar ratio up to 2 over Ni–Fe catalysts [28] and Cu/Nb2 O5 catalyst [32] previously in situ pretreated in H2 . It has been shown that H2 production can be increased by the addition of O2 , and that an optimum can be obtained with an O2 /EtOH ratio of 1.5 [28]. Very high conversion of ethanol could be obtained at 300 ◦ C (100% on Cu/Nb2 O5 and 87 % on Ni–Fe), and total conversion of O2 was observed at lower temperature (200 ◦ C on Cu/Nb2 O5 ). Therefore, the results obtained here are in good agreement with previous results obtained on different types of catalysts, even if products distributions could be different. In particular no CH4 on the Ni–Fe catalyst, and little formation of CO on Cu/Nb2 O5

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Fig. 6. Ethanol conversion and gas phase products distribution in POE with an O2 /EtOH = 1.5 over fresh (solid symbol) and treated in H2 at 250 ◦ C (hollow symbol) CeNi1 OY catalyst (0.2 g).

400

80

300

60 40

200

20

100

0

200

250

300

350

400

450

500

T (oven) (°C)

Conv. EtOH, prod. (%)

100

0

T (reaction) (°C) Fig. 7. Ethanol conversion () in the presence of water and oxygen (EtOH/H2 O/O2 /N2 = 1/3/1.2/N2 and EtOH = 14 mol%), and gas phase products distribution on CeNi1 OY catalyst (0.2 g) in situ treated in H2 at 250 ◦ C. H2 (♦), CO2 (), CH3 CHO (䊐), CH4 () and CO (䊉), and oven temperature (+).

were observed. Subramani et al. proposed that the large amounts of H2 formation in POE could be explained by the fact that the acetaldehyde formed by the oxidative dehydrogenation could be partially oxidized to H2 and carbon oxides [7]. 3.3. Oxidative steam reforming of ethanol (OSRE) The conversion is largely increased when adding O2 to the reaction mixture compared to SRE conditions. As an example, Fig. 7 presents the results obtained in OSRE conditions over the CeNi1 OY catalyst pretreated in H2 at 250 ◦ C (using harsh conditions: 14 mol% EtOH). For comparison, about 40% of ethanol conversion was obtained at 250 ◦ C in SRE conditions with the same mass of catalyst (Fig. 2). At a reaction temperature of 250 ◦ C, the ethanol conversion is very high at about 75% with about 40% of H2 in the gas phase products. At 400 ◦ C, ethanol is totally converted. As expected, CO2 is found in high quantity while CO presents an optimum at 350 ◦ C. CH4 increases with temperature whereas acetaldehyde disappears at about 400 ◦ C. As already reported, dehydrogenation of ethanol, produces acetaldehyde and H2 [Eq. (6)] while decompo-

sition of ethanol (and/or acetaldehyde) produces methane, carbon monoxide and hydrogen [Eq. (7)]. CO can react with water in the so called WGS reaction. A stable activity i.e., ethanol conversion, and H2 selectivity can be obtained at very low temperature when the solid is previously in situ treated in H2 at 250 ◦ C. So, a beneficial effect of the H2 pretreatment is also observed as already reported for SRE and POE reactions. However, it is important to remark that in presence of O2 , once the catalytic reaction started at 200 ◦ C only a small energy supply was needed. The oven temperature was lowered to about 130 ◦ C. So the results reported at a reaction temperature of 250 ◦ C are obtained with a much lower oven temperature, as previously observed in POE when using a high concentration of EtOH. The evolution of the reaction temperature is reported versus the oven temperature i.e., the energy needed to maintain the catalytic reaction. The catalytic results could be ameliorated and high activity has been reported using only a small quantity of catalyst (0.03 g) [59,60]. The unique activation phenomenon of the reaction (the dramatic variation of temperature between the catalyst bed and the oven) was deeply analyzed previously [59]. Moreover, the remarkable catalytic stability was attributed to the graphitic filamentous carbon formed during the reaction. Fig. 8 shows full ethanol conversion on the CeNi1 OY catalyst with a H2 formation of about 45% (mol%) relative to all the gas phase products (dry basis). The other products analyzed are mainly CO2 (41%) and CO (12%) with very low formation of CH4 and CH3 CHO (<1%). Continuous complete conversion of ethanol specifically at 50 ◦ C (oven temperature), with simultaneously production of H2 , is obtained. The catalyst exhibits remarkable stability after at least 75 h of reaction, even if carbon deposition is found after the reaction. Therefore, compared to previous results we reported, the oven temperature has been still decreased (50 ◦ C compared to 60 ◦ C previously reported). Finally, a cerium nickel based catalyst was developed for the highly efficient sustainable H2 production from ethanol and water in the OSRE reaction involving only a small energy input. The catalyst is able to continuously completely convert ethanol specifically at 50 ◦ C (oven temperature), and simultaneously produce H2 , CO2 and CO. To the best of our knowledge, OSRE at a so low temperature

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Fig. 8. Ethanol conversion and products distribution in the presence of water and oxygen (EtOH/H2 O/O2 /N2 = 1/3/1.6/1.3; EtOH = 14 mol%) with an oven temperature fixed at 50 ◦ C on CeNi1 OY catalyst (0.03 g) in situ treated in H2 at 250 ◦ C (the reaction has been started at 200 ◦ C).

7

Fig. 9. Raman analysis of CeNi1 OY compound and ceria from ref [60] is reported for comparison.

Table 1 Ni content, specific surface area and crystallites size of CeNi1 OY catalyst. Catalyst

Ni (wt.%)

SBET (m2 g−1 )

d CeO2 (nm)

d NiO (nm)

CeNi1 OY

20

94

5

10

has not been reported before. Sato et al. reported OSRE at very low temperature (100 ◦ C) on Ni/Ce0.5 Zr0.5 O2 and Ni/CeO2 catalysts [40]. They have shown that reduced Ni/Ce0.5 Zr0.5 O2 repeatedly triggered OSRE of ethanol at 100 ◦ C even in the presence of a stoichiometric excess of steam. Interestingly, the furnace heater was switched off after the reaction mixture was fed. However, the conversions and H2 formation rates were measured after 30 min, but in repeated cycles. In the OSRE process, the endothermic SRE is assisted by the exothermic POE reaction. The competing reactions such as dehydrogenation of ethanol to acetaldehyde [Eq. (6)], and ethanol decomposition to CH4 , CO and H2 [Eq. (7)] could also occur. It has been reported that OSRE could occur to produce H2 and CO/CO2 as major products with maximum H2 yield above 600 ◦ C, if CH4 is formed as an intermediate. Therefore, usually, the OSRE process has been studied at high temperatures and low O2 /EtOH ratios [50–52]. However, a catalyst that has poor selectivity for ethanol decomposition to CH4 could produce H2 and CO2 at relatively low temperature, because thermodynamic analyses also indicated that when acetaldehyde is formed as an intermediate, it could be readily converted into H2 and CO2 via oxidative steam reforming; this reaction being highly favorable would go to completion even at low reaction temperatures. Selected best performing catalysts for the low temperature OSRE (300–450 ◦ C) have been proposed, and it has been suggested that these catalysts proceed without involving CH4 as intermediate [7].

Fig. 10. XRD in H2 (10 h in H2 flow) at different reduction temperatures for CeNi1 OY catalyst. (a) 25 ◦ C, (b) 200 ◦ C, (c) 250 ◦ C, (d) 266 ◦ C, (e) 300 ◦ C, (f) 450 ◦ C. CeO2 ( ), NiO (), Ni0 (䊉).

Fig. 11. TPR profile of CeNi1 OY compound.

3.4. Characterizations It is well known that physicochemical properties of a catalyst play an important role in the evolution of surface reactions. The main factor leading to a high activity i.e., ethanol conversion, and H2 selectivity at very low temperature is the in situ activation of the solid in H2 in a temperature range between 200 and 300 ◦ C. Different physicochemical techniques, including TPR, TGA, XRD, Raman, XPS, ion sputtering, and TEM, were used to characterize the CeNiX OY catalysts [63,70–72]. A relatively high surface area is obtained for the CeNi1 OY catalyst (Table 1). A cerium nickel solid solution and highly dispersed nickel oxide with ceria can be obtained for the compound. The Raman spectroscopy results show a strong frequency shift and broadening for the first-order F2g ceria peak located near 460 cm−1 related to fluorite nano-crystalline CeO2 (Fig. 9) [64–66]. This behavior is interpreted as a result of the

insertion of Ni into the ceria phase (Ni species replacing some Ce species), in agreement with the presence of a solid solution [67]. Ion sputtering followed by XPS analysis allowed estimating the size of NiO nanoparticles (2 nm) co-existing in the compound with some larger nanoparticles evidenced by XRD and microscopy [63]. As a matter of fact, XRD allowed an estimation of the crystallite size at about 5 nm for CeO2 like species and at about 10 nm for NiO species for the calcined compound (Fig. 10, Table 1). The TPR profile in H2 of the catalyst is shown in Fig. 11. A first reduction temperature peak is observed at 266 ◦ C. This peak is more intense for a low Ni content on CeNiX OY catalysts and has been attributed to the reducibility of Ni species in solid solution and/or small NiO nanoparticles [72]. The second peak at about 370 ◦ C that increases with the Ni content on CeNiX OY catalysts is attributed to larger NiO nanoparticles (10 nm) visible by XRD, in good agreement with literature [72].

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Fig. 12. INS spectrum of CeNi1 OY treated in H2 at 250 ◦ C (after subtraction of the spectrum obtained on CeNi1 OY treated in vacuum at 200 ◦ C).

The Ni species from the solid solution and/or small NiO particles are first concerned by the reduction when increasing temperature [Eq. (8)], and then those of the larger NiO particles but with the simultaneous re-oxidation of a part of these species by reduction of the Ce4+ ions in their vicinity into Ce3+ species [Eq. (9)] as the existence of a redox system was established [63,70,71]. For an in situ treatment temperature up to 300 ◦ C in H2 , metallic Ni is not observed by in situ XRD in H2 (Fig. 10), therefore a “partially reduced” solid is obtained, with mainly the generation of anionic vacancies, which number is increased by the H2 treatment. Ni2+ + O2− + H2 → Ni0 + H2 O + 䊐

(8)

(with 䊐: anionic vacancy) 2Ce4+ + Ni0 ↔ 2Ce3+ + Ni2+

(9) 250 ◦ C,

the Moreover, during the activation treatment in H2 at anionic vacancy is filled with hydride species by heterolytic splitting of H2 [Eq. (10)]. O2− Mn+ 䊐 + H2 → OH− Mn+ H−

(10)

As shown by INS (Fig. 12). Two new intense and large bands at about 460 and 870 cm−1 are observed on the solid treated in H2 , due to the insertion of different hydrogen species. The hydrogen species of the hydride nature related to the peak at about 460 cm−1 appear clearly after a treatment in H2 at 250 ◦ C [68]. The band at 870 cm−1 has been previously attributed to hydrogen species related to the presence of metallic Ni [68]. However, it has to be recalled that metallic Ni is not observed by in situ XRD in H2 at this temperature (Fig. 10). Therefore, during the activation treatment in H2 in temperature, the CeNi1 OY solid becomes a hydrogen reservoir with hydrogen species stored in the bulk and at the surface of the solid; anionic vacancies are created, able to accept hydride species with the formation of an oxyhydride [69]. 3.5. Modeling Particular active sites were modeled, involving anionic vacancies, hydrogen species and cations in strong interaction whatever in the solid solution or at the NiO and ceria, and/or solid solution (with Ni) interfaces. Different kinds of active sites which differ from each other in terms of the environment of Ni can exist [69]. Therefore, H2 production from ethanol in presence of water (and oxygen) certainly involves particular Ni species in strong interaction with Ce species. It is clearly shown by INS that the activation treatment in H2 allows the cerium nickel based mixed oxide to become an oxyhydride in agreement with previous results reporting that this catalyst is able to accept large quantities of hydrogen in the bulk and at the surface of the solid by the presence of anionic vacancies [70]. Taking into account that dehydrogenation step requires abstraction of hydrogen species from alcohol, the ability of the solid

Scheme 1. X M–Y M active site generated in H2 on cerium and nickel solid solution with oxyhydride formation and ethanol activation by heterolytic abstraction of a hydride species. (with 䊐: anionic vacancy, and Nin+ = Ni2+ , Niı+ and Cem+ = Ce4+ , Ce3+ ). As an example a 3 M–3 M is represented.

to accept hydrogen and to allow its mobility can be an important factor. By analogy to the heterolytic dissociation of H2 , heterolytic dissociation of ethanol can be envisaged on a low coordination site involving anionic vacancies as it has already been proposed for oxidative dehydrogenation of propane [71] and methane activation [72] on CeNiX OY compounds. The active nickel species belonging to the small particles and/or to the solid solution, participating actively to the catalytic reaction, present the characteristic of being able to be reduced and re-oxidized easily and reversibly (redox process), allowed by their strong interaction with Ce species. As an example, Scheme 1 presents such a site that can be obtained in the solid solution of cerium and nickel, where the Ni cation is replacing a Ce cation inside the ceria phase. The active site is modeled by an ensemble of two cations (nickel–cerium) in strong interaction, which can be also generally expressed by X Ni–Y M (where x and y are the unsaturation degrees of each cation), as it has been proposed previously on Ni based catalysts [69]. The possible mechanism for ethanol transformation can be envisaged taking into account the X Ni–Y M ensemble. For example, with a lower number of anionic vacancies on the site (1 Ni–1 M), acetaldehyde and H2 [Eq. (6)] can be produced by heterolytic abstraction of hydrogen from ethanol [Eq. (11)] [59]. After in situ pretreatment in H2 the solid is in the oxyhydride form, it is partially reduced with the presence of occluded hydrogen species which allow H2 formation at low temperature even in presence of O2 [Eq. (12)]. Each elementary X Ni–Y M ensemble is associated with a particular reaction. Depending on the unsaturation degree of the active site, conversion of ethanol can lead to different products. The 3 Ni–1 M site can lead to the formation of H2 , CO and CH4 [Eq. (7)] [59]. This model also presents the advantage to be in good agreement with the synergetic effect observed when several cations with strong interactions are in presence in a mixed oxide. O2− Mn+ 䊐 + C2 H5 OH → OH− Mn+ H− + C2 H4 O −

n+

OH M

−

−

H + C2 H5 OH → OH M

n+

−

H + H2 + C 2 H4 O

(11) (12)

By using the exothermic reaction between hydride species of the solid and O2 [Eq. (13)] (chemical energy) together with the exothermic reaction between ethanol and O2 (partial oxidation), the reaction can be performed at particularly low temperature. 1 H− + O2 → OH− 2

(13)

Moreover, it has been proposed that, depending on the O2 concentration, the hydride can react and lead to CH4 formation [59]. In such a case partial oxidation of methane leading to H2 and CO cannot be discarded [73]. However, no CH4 is observed with the high O2 /EtOH ratio (Fig. 8), in agreement with easy acetaldehyde transformation in the presence of water and oxygen [Eq. (14)] [7].

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Besides, a high number of anionic vacancies, for example on the and 3 Ni–3 M site, can be obtained by the treatment in H2 at 250 ◦ C [69]. Such sites can be easily obtained on the edges/corners of the particles. Therefore, there can be enough anionic vacancies on a site to dissociate also water and/or to abstract all the H atoms from ethanol. It has been shown that when CHX species are formed all C H bonds break in one step and not on O atoms [73]. On this kind of sites a mixture of ethanol water and oxygen can be converted into H2 and CO2 . 3 Ni–2 M

C2 H4 O + H2 O + O2 → 2CO2 + 3H2

(14)

4. Conclusion Interesting hydrogen production from ethanol has been obtained in SRE, POE, and OSRE at very low temperature on the CeNi1 OY catalyst by applying accurate activation of the catalyst in H2 and by varying reactants concentration. The treatment of the catalyst in H2 at adequate temperature has a beneficial effect on ethanol conversion and hydrogen production at low temperature. In partial oxidation of ethanol, conversion reaches more rapidly 100% at lower temperature (350 ◦ C) with the highest O2 /EtOH ratio studied (1.5). The characterizations of the solid show that the active nickel species are in strong interactions with Ce species, allowing a redox process. Moreover, the existence of a hydride species is clearly shown by INS and during the treatment in H2 at 250 ◦ C the solid becomes an oxyhydride. Correlations among the species present in the solid, their reducibility and the catalytic performances are discussed and an active site is proposed. Acknowledgments W. Fang gratefully acknowledges a grant from Erasmus Mundus Tandem. C. Pirez gratefully acknowledges a grant from French ministry. The authors thank ILL (Institut Laue Langevin, France) for supporting the INS experiments and the help of ILL group. The authors would like to thank Ms. L. Burylo (for XRD), Mr. O. Gardoll (for TPR), and Mr. J. C. Morin (for Raman spectroscopy) from Unité de Catalyse et Chimie du Solide. Chevreul institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Nord-Pas de Calais and FEDER are acknowledged for supporting and funding this work. References [1] M. Momirlan, T.N. Veziroglu, Int. J. Hydrogen Energy 30 (2005) 795. [2] A. Midilli, M. Ay, I. Dincer, M.A. Rosen, Renew. Sustain. Energy Rev. 9 (2005) 273. [3] A. Haryanto, S. Fernando, N. Murali, S. Adhikari, Energy Fuels 27 (2005) 1125. [4] P.D. Vaidya, A.E. Rodrigues, Chem. Eng. J. 117 (2006) 39. ˜ J.L.G. Fierro, Chem. Rev. 107 (10) (2007) 3952. [5] R.M. Navarro, M.A. Pena, [6] M. Ni, D.Y.C. Leung, M.K.H. Leung, Int. J. Hydrogen Energy 32 (2007) 3238. [7] V. Subramani, C. Song, Catalysis 20 (2007) 65. [8] P. Ramírez de la Piscina, N. Homs, Chem. Soc. Rev. 37 (2008) 2459. [9] L.V. Mattos, G. Jacobs, B.H. Davis, F.B. Noronha, Chem. Rev. 112 (2012) 4094. [10] N. Bion, D. Duprez, F. Epron, ChemSusChem 5 (2012) 76. [11] S. Li, J. Gong, Chem. Soc. Rev. 43 (21) (2014) 7245. [12] S. Cavallaro, V. Chiodo, S. Freni, N. Mondello, F. Frusteri, Appl. Catal. A 249 (1) (2003) 119. [13] F. Frusteri, S. Freni, V. Chiodo, L. Spadaro, G. Bonura, S. Cavallaro, J. Power Sources 132 (2004) 139. [14] L.E. Arteaga, L.M. Peralta, V. Kafarov, Y. Casas, E. Gonzales, J. Chem. Eng. 136 (2008) 256. [15] C. Resini, M.C.H. Delgado, S. Presto, L.J. Alemany, P. Riani, R. Marazza, G. Ramis, G. Busca, Int. J. Hydrogen Energy 33 (2008) 3728. [16] M. Liao, W. Wang, R. Ran, Z. Shao, J. Power Sources 196 (2011) 6177. [17] D. Srinivas, C.V.V. Satyanarayana, H.S. Potdar, P. Ratnasamy, Appl. Catal. A 246 (2003) 323. [18] P. Biswas, D. Kunzru, Int. J. Hydrogen Energy 32 (2007) 969. [19] G. Zhou, L. Barrio, S. Agnoli, S.D. Senanayake, J. Evans, A. Kubacka, M. Estrella, J.C. Hanson, A. Martínez-Arias, M. Fernández-García, J. Rodriguez, Angew. Chem. Int. Ed. 122 (2010) 1.

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